Rolling friction is the resistance to motion experienced by a body when it rolls upon another. It is much less than sliding friction for same pair of bodies. When one body rolls upon another, there is theoretically no sliding or slip between them. And if both are perfectly rigid, there is no surface of contact. 

Rolling friction is considerably less than sliding friction as there is no work done against the body that is rolling by the force of friction. For a body to start rolling a small amount of friction is required at the point where it rests on the other surface, else it would slide instead of roll.

Reasons why rolling friction is less than sliding friction:
- Surface Contact:
- Rolling friction occurs when an object rolls over a surface, resulting in less surface contact compared to sliding friction, where two surfaces slide against each other. The reduced contact area in rolling friction leads to less resistance.
- Energy Dissipation:
- In sliding friction, more energy is dissipated as heat due to the continuous rubbing of surfaces against each other. This energy loss contributes to higher frictional forces compared to rolling friction, where energy dissipation is lower.
- Mechanism of Friction:
- Rolling friction involves a combination of rolling and static friction, which are generally lower than the kinetic friction experienced in sliding friction. The rolling motion helps in reducing resistance and requires less force to maintain motion.
- Surface Irregularities:
- Sliding friction is affected by surface irregularities and imperfections, leading to higher frictional forces. In contrast, rolling friction is less affected by surface conditions due to the rolling motion, resulting in lower resistance.
- Lubrication:
- Rolling friction can be reduced further by using lubricants or ball bearings, which help in minimizing contact between surfaces and reducing friction. This lubrication mechanism is not as effective in sliding friction scenarios.
In conclusion, the combination of less surface contact, lower energy dissipation, the mechanism of friction, reduced impact of surface irregularities, and potential for lubrication all contribute to rolling friction being less than sliding friction.